Analog-to-digital converters (ADCs) are widely used to convert analog voltage signals into digital output signals. ADCs are often embedded with other components in integrated circuits, for example, in microcontrollers. The performance of ADCs that are embedded in integrated circuits is often worse than the performance of standalone ADCs with the same specification. The presence of noise on signal lines and power supply lines often results in higher signal interference and output errors in embedded ADCs. Another source of errors in ADCs is the nonlinearity of the transfer functions of the ADCs. Thus, calibrating embedded ADCs to correct for errors can significantly enhance performance.
Ideally, the Vin versus Dout transfer function of an ADC is a straight line. A linearly increasing voltage amplitude is ideally converted by the ADC into a linearly increasing digital value. The extent to which an ADC receives an analog input signal with a linearly increasing voltage amplitude and outputs digital values that do not linearly increase in value is referred to as “integral nonlinearity error” (INL error). An ADC exhibits differential non-linearity (DNL) error, the derivative of INL error, where it outputs discontinuous digital values in response to a smoothly changing voltage input. DNL error is the extent to which the digital values output by an ADC do not increase by steps of 1 LSB as the analog input voltage increases gradually. An ADC ideally converts an input of the minimum voltage into a digital value of all zeros. The extent to which the digital output value deviates from digital zero is referred to as “dc offset error.” An ADC ideally converts an input of the maximum voltage into a digital value of all ones. The extent to which the digital output value output deviates from all digital ones is referred to as “gain error.”
There are various known methods for calibrating ADCs to correct for gain error, dc offset error, INL error and DNL error. In one method, the uncorrected digital output of an ADC with a nonlinear transfer function is mapped to the ideal linear transfer function. The calibration method determines a correction factor for each uncorrected digital value output by the ADC and stores the uncorrected value and its associated correction factor in a lookup table. A calibration circuit outputs a corrected digital value on the ideal linear transfer function by adding the appropriate correction factor to each uncorrected digital value output by the ADC. Valuable memory resources must be allocated to the lookup table. A calibration method that consumes less memory resources for a lookup table would be particularly advantageous in an ADC that is embedded in a microcontroller.
Another calibration method involves piecewise linear segmentation. The entire dynamic range of the uncorrected output of an ADC is divided into equally-spaced ranges of digital values. A best-fitting linear segment is then mapped to the distribution of digital values within in each range. Each uncorrected digital output value is associated with a point on a linear segment. Correction factors are then calculated for each point along each linear segment. The output of the ADC is corrected by adding to each uncorrected digital value the correction factor associated with the closest point on the appropriate linear segment.
A calibration method that divides an uncorrected nonlinear transfer function into segments of approximately equal length is less accurate for segments where the uncorrected transfer function is most nonlinear. Moreover, depending on the shape of the uncorrected transfer function, calculating correction factors with additional segments does not necessarily render the calibration more accurate. For an uncorrected, hockey-stick-shaped transfer function, for example, approximating the uncorrected transfer function with successive linear segments along the handle does not provide a better fit than would a single linear segment. Calculating correction factors for the successive linear segments does not significantly improve the resolution of the ADC.
A method is sought for calibrating an analog-to-digital converter that reduces the memory used to store correction factors and that also reduces any calculations that do not significantly improve the resolution of the analog-to-digital converter.